1. Field of the Invention
Aspects of the invention relate to a mobile communication method of performing beamforming with respect to a terminal using a multi-antenna of a base station, and more particularly to a method of constructing a codebook, which is a group of beamforming vectors, and a method of selecting a beamforming vector from the codebook.
2. Description of the Related Art
Multi-user diversity is a scheme for communicating between a base station and a plurality of mobile terminals served by the base station that concentrates on transmitting data to only ones of the mobile terminals that have an excellent wireless channel state between the base station and the ones of the mobile terminal, and can thereby improve data transmission efficiency.
When a wireless channel state between a base station and a mobile terminal is excellent, data can be transmitted at a higher transmission speed. Conversely, when the wireless channel state between the base station and the mobile terminal is poor, the data transmission speed may be limited by the wireless channel, causing data to be transmitted at a comparatively lower transmission speed. Due to movement of the mobile terminal, fading, and the like, the wireless channel state varies over time. Accordingly, although the mobile terminal may have a poor wireless channel state at one point in time, it may have an excellent wireless channel state at another point in time.
Accordingly, if the base station can know the wireless channel state between the base station and the mobile terminal, the base station can concentrate on transmitting data to only a mobile terminal having an excellent wireless channel state, and thus the overall data transmission speed in the base station can be improved.
In order to provide an improved transmission speed between a base station and a mobile terminal, research is underway on a Multiple-Input Multiple-Output (MIMO) communication system in which the base station and the mobile terminal each have a plurality of antennas, and transmit and receive data using the plurality of antennas.
When using an independent wireless channel provided between the plurality of antennas, diversity gain can be achieved and thus a fading effect can be overcome. Also, when downlink beamforming is performed using an appropriate beamforming vector before transmitting data using the plurality of antennas, it is possible to concentrate on transmitting data to a mobile terminal located in a particular direction, and to restrict transmission of a signal to other mobile terminals located in other directions. Accordingly, an interference effect between downlink signals transmitted to a mobile terminal through downlink beamforming can be reduced, making it possible to transmit data at a high speed.
However, in the case of the downlink beamforming scheme, a number of nulls to eliminate the downlink interference is defined by a number of degrees of freedom, which is determined by a number of transmitting antennas. Accordingly, it is difficult to apply the downlink beamforming scheme in a general mobile communication environment in which a number of mobile terminals served by the base station is much greater than a number of transmitting antennas of the base station.